The international applications WO98/21208 (=U.S. Pat. No. 6,008,215), WO98/40382 (=U.S. Pat. No. 6,143,759), WO99/57118 (=U.S. Pat. No. 6,306,869) and WO00/12501 describe 6-phenylbenzonaphthyridines and their N-oxides as PDE3/4 inhibitors.